With the aid annotated diagrams, explain the characteristics and mode of formation of the following features: (a) gorges (b) self dunes

(a) Gorge

Characteristics: A gorge is a narrow, steep-sided (almost vertical) and deep river valley. It is usually cut in hard, resistant rock, has a fast-flowing river occupying the whole floor, and often occurs below a waterfall or in a limestone/arid region.

Mode of formation: A gorge is formed mainly by rapid vertical (downward) erosion by a river where the rock is hard and the sides do not weather back quickly. It may form by the retreat of a waterfall upstream, by river rejuvenation, or by the collapse of the roof of an underground cavern in a limestone area.

Annotated diagram: Draw a cross-section of a very narrow, deep valley with near-vertical walls and a river filling the floor; label "vertical/steep sides", "hard resistant rock", and "river occupying floor".

(b) Seif dune

Characteristics: A seif (longitudinal) dune is a long, narrow ridge of sand lying parallel to the prevailing wind direction. It has a sharp crest, may run for many kilometres, and occurs in groups separated by wind-swept corridors of bare rock.

Mode of formation: Seif dunes form in deserts where a strong, steady prevailing wind blows sand along, and occasional cross-winds from a slightly different direction sweep the sand into long parallel ridges aligned with the dominant wind. Continuous saltation drags the sand grains forward along the ridge.

Annotated diagram: Draw a plan/oblique view of long, straight, parallel sand ridges; add a large arrow showing the prevailing wind blowing along the length of the ridges, and label "crest", "parallel to wind", and "corridor of bare rock between dunes".